Photosensitized generation of singlet oxygen from (substituted bipyridine)ruthenium(II) complexes

Photophysical properties in dilute MeCN solution are reported for seven Ru- II complexes containing two 2,2'-bipyridine (bpy) ligands and different thi rd ligands, six of which contain a variety of 4,4'-carboxamide-disubstitute d 2,2'-bipyridines, for one complex containing no 2,2'-bipyridine, but 2 of these different ligands, for three multinuclear Ru-II complexes containing 2 or 4 [Ru(bpy),] moieties and also coordinated via 4,4'-carboxamide-disub stituted 2,2'-bipyridine ligands, and for the complex [(Ru(bpy)(2)(L)](2+) where L is N.N'-([2,2'-bipyridine]-4,4'-diyl)bis[3-methoxypropanamide]. Abs orption maxima are red-shifted with respect to [Ru(bpy)(3)](2+), as are pho sphorescence maxima which vary from 622 to 656 nm. The lifetimes of the low est excited triplet metal-to-ligand charge transfer states (MLCT)-M-3 in de -aerated MeCN are equal to or longer than for [Ru(bpy)(3)](2+) and vary con siderably, i.e., from 0.86 to 1.71 mus. Rate constants k(mu) for quenching by O-2 of the (MLCT)-M-3 states were measured and found to be well below di ffusion-controlled. ranging from 1.2 to 2.0 (.) 10(9) dm(3) mol l s (1). Th e efficiencies f(Delta)(T) of singlet-oxygen formation during oxygen quench ing of these (MLCT)-M-3 states are relatively high, namely 0.53 - 0.89. The product of k(q) and f(Delta)(T) gives the net rate constant k(q)(1) for qu enching due to energy transfer to produce singlet oxygen, and k(q) - k(q)(1 ) equals k(q)(3), the net rate constant for quenching due to energy dissipa tion of the excited (MLCT)-M-3 states without energv transfer. The quenchin g rate constants were both found to correlate with DeltaG(CT), the free-ene rgy change for charge transfer from the excited Ru complex to oxygen, and t he relative and absolute values of these rate constants are discussed.